I use path tracing for scientific/engineering studies of light propagation; in particular, I attempt to simulate monochromatic light sources and reflective/transmissive/absorptive material configurations (ranging from specular to diffusive, and everything in between) to determine the irradiance delivered to specific geometries. In the past I have used commercial packages like ASAP (APEX Solidworks add-in), Zemax, and FRED (in order of preference).